1. Field of the Invention
The present invention relates to a method for manufacturing a liquid jet recording head, and more particularly to a method for manufacturing a liquid jet recording head having thermal energy generation means.
2. Related Background Art
Of the known recording methods, a liquid or ink jet recording (ink jet recording method is a non-impact recording method which does not generate noise in recording characters, enables high speed recording, and can record characters on a plane paper without special fixing process is very effective. Various proposals have been made to the liquid jet recording method and some of them have been commercialized and some of them are still under study.
In the liquid jet recording method, droplets of the recording liquid (ink) are flown by one of several actions and they are deposited to a record sheet such as a paper to record the characters.
The applicant of the present invention has proposed a novel liquid jet recording method in, for example, German Patent application No. DE284306401A1. A basic principle thereof is as follows. A thermal pulse is applied as an information signal to recording liquid in an action chamber so that the recording liquid generates vapor bubbles and self-shrinks. By a force created during the above process, the recording liquid is discharged from a liquid discharge port connected to the action chamber so that it flies as droplets, which are deposited on the record sheet to record the characters.
In this method, by using a high density multi-array structure, high speed recording and color recording are easily attained, and the construction of the apparatus is simpler than a conventional one. Accordingly. a recording head is compact and suitable for mass production. By fully utilizing advantages of IC technology and micro-machining technology which have been well developed in a semiconductor field, a long web can be easily manufactured.
A typical recording head of a liquid jet recorder used in the above liquid jet recording method is provided with thermal energy generation means for discharging recording liquid from a liquid discharge port to form flying droplets.
The thermal energy generation means is preferably arranged to directly contact the recording liquid so that the generated thermal energy effectively acts on the recording liquid and an ON-OFF response speed of the thermal action on the recording liquid is increased.
However, the thermal energy generation means basically comprises a heat generating resistive layer which generates heat when energized and a pair of electrodes for supplying a power to the heat generating resistive layer. Accordingly, if the heat generating resistive layer directly contacts the recording liquid, the recording liquid is electrolyzed by a current flowing through the recording liquid depending on an electrical resistance of the recording liquid, or the heat generating resistive layer reacts with the recording liquid when a current is supplied to the heat generating resistive layer so that the resistance of the heat generating resistive layer changes due to erosion thereof, or the heat generating resistive layer is broken or damaged.
In the past, the heat generating resistive layer has been made of an inorganic material such as NiCr alloy or metallic boronide such as ZrB.sub.2 or HfB.sub.2, which has a relatively excellent property as the heat generating resistive material, and a protection layer made of high anti-oxidization material such as SiO.sub.2 is formed on the heat generating resistive layer to prevent the heat generating resistive layer from directly contacting the recording liquid, in order to resolve the above problems and improve the reliability and durability for repetitive use.
In forming the thermal energy generation means of the liquid jet recording head, it is common to form the heat generating resistive layer on a substrate and then stack electrodes and a protection layer thereon. The protection layer of the thermal energy generation means must uniformly cover the heat generating resistive layer and the electrodes without defects such as pinholes so that it fully functions as the protection layer to prevent breakage of the heat generating layer and short circuits between the electrodes.
In the liquid jet recording head, the electrodes are usually formed on the heat generating resistive layer and hence there is a step between the electrode and the heat generating resistive layer. Since the layer thickness is uniform at the step, the layer must be formed to completely cover the step so that there is no exposed area. If the step coverage is not complete, the exposed area of the heat generating resistive layer directly contacts to the recording liquid so that the recording liquid is electrolyzed or the recording liquid reacts with the heat generating resistive layer to break the heat generating resistive layer. The film is not homogeneous at the step. Such non-homogeneity results in concentration of thermal stress in the protection layer through repetitive heat generation and can cause cracks in the protection layer. The recording liquid can penetrate through such cracks to break the heat generating resistive layer. Further, the recording liquid may penetrate through pinholes to break the heat generating resistive layer.
In the past, in order to resolve the above problems, the thickness of the protection layer is increased to improve the step coverage and reduce the pinholes However, the thick protection layer contributes to the improvement of the step coverage and the reduction of the pinholes but impedes the supply of heat to the recording liquid, which raises the following additional problem.
The heat generated in the heat generating resistive layer conveyed to the recording layer through the protection layer. When the protection layer is thick, the thermal resistance between the protection layer which is an action plane of the heat and the heat generating resistive layer increases and hence more power must be supplied to the heat generating resistive layer. Accordingly,
.circle.1 It is disadvantageous for power saving. PA1 .circle.2 Unnecessary heat is stored in the substrate and thermal response is lowered. PA1 .circle.3 Durability of the heat generating resistive layer is lowered because of larger power.
Those problems may be resolved by reducing the thickness of the protection layer. However, in the conventional method for manufacturing the liquid jet recording head in which a film forming method such as sputtering or vapor deposition is used to form the protection layer, there is a problem of durability because of insufficient step coverage and it is difficult to reduce the thickness of the protection layer.
In the recording by the liquid jet recording head, it has been known that forming stability of the recording liquid is improved as the recording liquid is heated more rapidly. Namely, the shorter the pulse width of an electrical signal (rectangular pulse) applied to the thermal energy generation means, the better is the forming stability of the recording liquid, and the discharge stability of the flying droplet and record quality are improved. However, in the conventional liquid jet recording head, the protection layer must be thick and hence the thermal resistance of the protection layer is high. As a result, a larger thermal energy must be generated by the thermal energy generation means and the durability and the thermal response are degraded. As a result, it is difficult to reduce the pulse width and the improvement of the record quality is limited.